The present invention relates to electric service load limiters, and more particularly, to an electric service load limiter for limiting the amount of electricity that is allowed to pass from a power line and through an electrical power leg supplying electricity to a consumer.
In the electric utility industry, household consumers pay for electric service according to the amount of energy consumed. Therefore, utility companies normally bill these customers in proportion to the total amount of electrical current drawn during a particular billing period. When such a customer becomes seriously delinquent in paying a bill, the utility company has the option of shutting off electric service until the bill is paid. While this usually is an effective incentive to cause a household consumer to make payments that are past due, it also presents potential regulatory problems. Local statutes often prohibit utilities from totally discontinuing service to the extent that a consumer is unable to operate essential appliances such as a furnace, a refrigerator, or a water pump. Regulations of this type typically prohibit discontinuing electrical service in the winter when operation of a furnace can be essential. For this reason, a utility company wishing to restrict a delinquent consumer""s electricity consumption must be able to do so without seriously disrupting the consumer""s essential electricity requirements.
Previous attempts to limit electrical consumption by delinquent household consumers have included mechanical circuit breaker-type load limiters. These limiters characteristically incorporate thermal or magnetically-triggered mechanisms that trip or automatically disconnect electrical power legs leading into a consumer""s building. The purely mechanical functioning of these designs often means that their proper operation requires constant, non-fluctuating electrical, physical or environmental conditions. For example, a transient current, such as is typically present in a home electrical circuit during the start of a major appliance, is sometimes sufficient to cause a mechanical load limiter to automatically disconnect, even though the total combined draw of all working electrical devices on the circuit when they are at equilibrium is well under the load limiter""s set point, or maximum allowed current draw. Excessive environmental heat can similarly affect the operation of thermally-triggered mechanical units. In a purely mechanical load limiter, the load limiter""s set point is also susceptible to change as the mechanism continues to wear over time, reducing the unit""s overall reliability.
To allow delinquent customers to continue drawing essential electricity requirements, mechanical service load limiters typically allow for customer resetting after automatic disconnection or tripping. A customer must manually restore service by resetting mechanical circuit breakers in the load limiters after an excessive, nonessential load has been removed. When a thermally actuated mechanical load limiter trips due to a customer""s excessive current draw, it is often difficult or impossible for a customer to immediately reset the load limiter because the overheated breaker mechanism remains hot for awhile. Thus, the consumer""s home may be without electricity for awhile. Additionally, due to their design, mechanical units are highly susceptible to customer tampering. For example, customers sometimes bypass a mechanical load limiter by simply taping or jamming a circuit breaker reset button into the reset position. In addition to rendering the unit inoperable, such tampering can also lead to overheating and damage, significantly reducing a load limiter""s service life.
A further disadvantage of existing mechanical load limiters has been the lack of selectable set point current levels on a single unit. Since previous load limiters depend on specific mechanical conditions being met before tripping, they are normally limited to a single set point level. As a result, a utility company must purchase and stock separate units for each different set point level it uses. It follows that a different load limiter must be installed for each individual customer according to the customer""s essential requirements. A utility company must also incur the added expense of installing a new unit when a new customer with different essential electric service requirements occupies the home of a previous customer or whenever an existing customer""s essential electricity requirements change. This also further precludes the possibility of setting a customer""s set point level remotely from a central location.
Previous attempts to limit electrical loads have also included the incorporation of solid state electronic monitoring and logic circuits for removing discrete loads from a customer""s power circuit after consumption surpasses a set-point maximum. U.S. Patent to Hedges (U.S. Pat. No. 4,211,933) discloses a method and U.S. Patent to Leyde (U.S. Pat. No. 4,034,233) discloses an apparatus and method for automatically limiting total current in a power circuit to a set-point maximum by sequentially disconnecting individual loads from a power circuit until total current drawn ceases to surpass the set-point maximum. Individual loads remain disconnected only as long as total current flowing through the power circuit continues to surpass the set-point maximum. Thereafter, disconnected loads are automatically reconnected to the power circuit, and no manual resetting by the consumer is required. At no time is all current on the building""s power circuit entirely disconnected from a utility power line. This substantially reduces a circuit""s effectiveness as a negative incentive for a consumer to make past due utility payments. Moreover, such circuits require a consumer to preset the priority in which loads are disconnected to selectively determine which loads are first terminated during an overload condition. In the event that load priority is improperly set or programmed, such an arrangement could effectively serve to terminate essential services while failing to terminate nonessential services or electrical conveniences to the consumer, thereby achieving the opposite of the desired result.
Other solid state devices specifically limit the amount of current that may travel through a consumer""s power circuit during a set interval. For example, U.S. Patent to Summe (U.S. Pat. No. 5,289,109) discloses a circuit including a current sensing resistor, a current-controlling P-channel field effect transistor and an NPN bipolar transistor. When the magnitude of a load current flowing through the circuit reaches a predetermined current limit value, the conduction of the current-controlling P-channel transistor is controlled to limit the load current flowing through the circuit. Current flowing through a power circuit is never permitted to exceed the set point maximum, regardless of the number of loads placed on a circuit. There is no trip mechanism and no means to automatically disconnect excessive individual loads. Thus, even in the event of an excessive total load, the design neither terminates nor automatically removes individual loads from the power circuit. While such devices may be suitable for imposing a strict limit on the amount of current that is permitted to be drawn through a power circuit, they are inherently unsuited for use as utility load limiters. In the absence of a trip or disconnect mechanism, such devices are alone incapable of signaling overload conditions. Rather than disconnecting loads in excess of a set-point maximum, such circuits merely distribute the limited current among the various loads that remain connected to the circuit, resulting in insufficient power to adequately maintain all connected loads. As a result, even essential appliances could be adversely affected, contrary to the desired effect of limiting the amount of power supplied to a consumer. Moreover, if a consumer becomes content with the substandard but barely functioning condition of one or more of his electrical appliances, the customer may be able to budget electricity consumption in a way that allows for continued delinquency without payment.
According to the principles of this invention, an electric service load limiter limits the level of current a consumer receives from an electric utility power line. The load limiter uses an electronically controlled power relay switch, connectable between the utility power line and the consumer electric power circuit, to couple electric current to the consumer""s power circuit and to interrupt the flow of current through the consumer power circuit when the level of current flow exceeds a predetermined maximum level. The load limiter has a sensor that produces a sensor signal which is proportional to the amount of current flowing through the consumer power circuit. The sensor signal may be amplified and/or converted into a direct current signal, or otherwise processed as desired, before being coupled to a logic circuit. The logic circuit compares the resulting sensor signal to a signal representing the maximum current level oft he consumer""s electric power circuit which was preselected by the utility. If the sensor signal reflects a level of current greater than the preselected maximum level, the logic circuit causes the automated power switch to disconnect the consumer""s electric power circuit from the utility power line, thereby disrupting electrical service to the consumer. A reset switch circuit is coupled to the power relay switch. The reset circuit produces a single reset signal each time the reset circuit is operated. This reset signal causes the automated power switch to couple current from the utility power line to the consumer""s electric power circuit.
In one embodiment of this invention, the reset circuit includes a manual switch which can be switched between a first condition and a second condition. The reset circuit produces a reset signal only when the manual switch is switched from the first condition to the second condition to cause the power relay switch to begin to couple current from the utility power line to the consumer""s electric power circuit. However, the manual switch must thereafter be switched from the second condition back to the first condition before the flow of current from the utility power line can be restored after the flow of current is interrupted once again. As a result, tampering with the manual switch to keep it in the posture which turns on the electric power cannot prevent the load limiter from interrupting electric power service.
Additionally, in another embodiment of this invention the reset switch circuit includes a manual switch that is connected to a capacitor that is separately charged for generating a reset signal while the switch is in a first condition. During normal operation, the capacitor is maintained in a charged stated by the separate power source. However, upon manual activation of the reset switch from its first condition to its second condition to connect the power leg to the consumer""s building, the capacitor is removed from its separate power source and electrically connected to the logic circuit. This electrical connection causes the capacitor to discharge, resulting in a reset signal being coupled through the logic circuit to turn on the power relay switches and restore electrical continuity along the one or more power legs, restoring electrical service. To function once again, the reset switch must be fully restored from its second condition to its first condition to recharge the capacitor before another reset signal can be generated to turn on the power-removing relays. Thus, taping or fixing the reset switch into the reset position is ineffective for preventing disruption of electrical service. A customer who continually draws beyond his essential requirements must therefore physically return to the inconvenient locations of the reset switch and fully activate it. This inconvenience becomes an increasing nuisance if the delinquent customer""s excessive usage of electricity continues.
In still another embodiment of the invention, the logic circuit includes a separate reference circuit for producing a set point voltage signal proportional to a desired predetermined maximum current level. The reference circuit may allow for variable settings so that different maximum current levels can be selected by the utility company.
One alternative embodiment of this invention achieves the effect of interrupting and resetting a consumer""s electric service without requiring the consumer to manually reset the service load limiter after each interruption. The reset circuit is a reset time delay circuit. The reset time delay circuit is coupled through the logic circuit to the automated power switch. In this alternative embodiment, a consumer also causes the power relay switch of the service load limiter to discontinue his electric power by drawing current beyond the predetermined maximum limit. However, the interruption of the electric current flow triggers the operation of the reset time delay circuit for a period of time which has been preselected by the utility company. During this period of time the consumer""s electric service is off. At the end of the delay period, the reset timing circuit produces a reset signal which causes the power relay switch to once again couple current from the utility power line to the consumer""s electric power circuit to restore the consumer""s electric service. However, the delay before electric service is automatically restored serves a nuisance function and provides a consumer with an incentive to either limit the consumption of electricity or pay the delinquent bill.
When the consumer""s electric power circuit being monitored includes more than one power leg onto each of which different appliances and equipment can be connected, a separate sensor can be used for each power leg. A sensor signal from each sensor can be compared with a signal representing the utility""s selected maximum current level. A logic circuit associated with each power leg can cause all of the power to the consumer to be discontinued by actuating the power relay switches if it determines too much current is being drawn through one of the power legs. Additionally, the signal representing the maximum current level can be produced by a reference circuit having a plurality of settings, each representing a separate, selectable maximum current level. This reference circuit can be used where the service load limiter monitors the current flowing through one power leg or more than one.
Additionally, another alternative embodiment of this invention allows the operational mode of the load limiter to be selected so that the load limiter either (1) turns on the power relay switch so there is no restriction on the amount of power flowing through the power leg, or (2) limits current to the level of a predetermined magnitude as established by a referenced circuit, or (3) terminates all current flow by shutting off the power relay switch.
The load limiter of this invention can be made from solid state electrical and electronic components. These components enable the load limiter to be readily controlled from a remote location, if desired. Thus, through the use of an electronic communication device, such as a modem, one of the three operational modes of the load limiter can be selected from a remote location. Additionally, the setting of a reference circuit can be remotely adjusted to select a desired maximum current level for a particular consumer. Where a reset time delay circuit is used, the length of the interval between the time when the automated power switch disconnects electric current from a consumer power circuit and the time when electric service restored can also be adjusted from a remote location.
This invention does not reside in any one of the features of the service load limiter that are disclosed above and described in the Description of the Preferred Embodiments. Rather, this invention is distinguished from the prior art by its particular combination of features of the load limiter claimed below. Important features of the invention have been disclosed in the Detailed Description of the Preferred Embodiments as shown and described below to illustrate the best mode contemplated to date for carrying out this invention.
Those skilled in the art will realize that this invention is capable of embodiments which are different from those shown and that details of the structure of the service load limiter can be changed in various manners without departing from the scope of this invention. Accordingly, the drawings and descriptions are to be regarded as illustrative in nature and are not to restrict the scope of this invention. Thus, the claims are to be regarded as including such equivalent electric service load limiters as do not depart from the spirit and scope of this invention.